Structure and significance of C30 4-methyl steranes in lacustrine shales and oils

Add time:08/31/2019    Source:sciencedirect.com

The structure of the abundant C30 4-methyl steranes of ancient freshwater lacustrine sediments and derived oils is of interest as their precursors, the 4-methyl and, in particular, 4α,23,24-trimethyl, sterols (dinosterols) are reported to be specific to dinoflagellates. This study employs a novel liquid crystal GC phase combined with GC-MS and GC-MS/MS to separate and characterise mixtures of C30 4-methyl sterane standards not resolvable on non-polar phases. This technique is then applied to identify the C30 4-methyl steranes in an ancient freshwater sediment, a freshwater lacustrine—and a marine—derived oil. A comparison is also made with the 4-methyl steranes in marine sediments containing varying amounts of dinoflagellate cysts.The C30 4-methyl steranes of the freshwater lacustrine samples have been shown to possess a 4-methyl-24-ethylcholestane structure, while both 4-methyl-24-ethylcholestanes and 4α,23,24-trimethyl-5α(H)-cholestanes (dinosteranes) were found in the marine dinoflagellate-rich sediments. While still indicative of dinoflagellates 4-methyl-24-ethyl sterols are generally not the major dinoflagellate sterol and alternative explanations for the abundance of 4-methyl-24-ethylcholestanes in ancient lacustrine sediments should be considered. Possibly ancient freshwater dinoflagellates synthesised principally the 4-methyl-24-ethyl form. Alternatively, a source from an unrecognized algal class is possible while a source from bacteria, which are known to synthesise certain 4-methylsterols, is discussed.

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